The “New” Charge Symmetry precision experiments in few nucleon systems

1. The “New” Charge Symmetry precision experiments in few nucleon systems from meson-exchange to effective field theory SPIN 2006 Kyoto, Japan Ed Stephenson Indiana University Cyclotron Facility How do we understand charge symmetry breaking? examples from meson exchange scattering lengths n-p analyzing power differences Keys to experimental precision examples from effective field theory n+p→d+π0 fore-aft asymmetry d+d→4He+π0

2. Traditional view of charge symmetry: Charge symmetry requires that any process/property be invariant under neutron-proton swap. (Not charge conjugation) Not considered here: isospin multiplets of baryons/mesons mirror nuclei Electromagnetic effects completely violate charge symmetry. We may still ask whether the strong interaction obeys this symmetry. Things which may violate charge symmetry: mass differences (n–p, π±–π0, …) meson isospin mixing (ρ0–ω, π0–η, …) (residual EM effects)

3. Charge Symmetry Breaking in the N-N Interaction mN– mP = 1.293 MeV How are these effects be understood within a meson exchange picture? Different NN scattering lengths app (corrected) fm -18 CSB ρ0 – ω mixing ann contribution of the π± - π0 mass difference in OPEP and TPEP -20 Charge Independence Breaking EM contributions in π+γ exchange -22 -24 anp

4. n-p analyzing power difference Layout of TRIUMF experiment n p CS p n . . n p n p Keys: beam and target polarized, same geometry for both cases look near zero crossing to avoid calibration issues

5. Results from IUCF and TRIUMF experiments residual EM effect comes from neutron magnetic moment in magnetic field of moving proton additional effect result of ρ0–ω mixing most of effect is attributed to n-p and pion mass differences

6. Shift in approach to sources of CSB: emphasize quark level sources rather than nucleon level swap u d u u d d neutron proton Charge symmetry requires that no process/property depend on the swap of the down and up quarks (in strong force). Use effective field theory to define contribution to CSB. New experiments involve pion production.

7. n+p→d+π0 [PRL 91, 212302 (’03)] Keys: all angles recorded in SASP at once detector efficiency independently calibrated data compared to Monte Carlo simulation with Afb variable Charge symmetry requires no change when n and p are swapped, so cross section is symmetric about 90°. SASP spectrometer focal plane detectors particles move through system target point

8. The data forward deuterons This difference is an artifact of the SASP. So asymmetry requires a detailed model of the experiment. backward deuterons All model properties determined independently, except: beam energy central SASP momentum target thickness A1/A0 = 2Afb Results from Monte Carlo study: Afb = 0.172 ± 0.080 ± 0.055 % (stat) (sys) where

9. d+d→4He+π0 [PRL 91, 142302 (’03)] Keys: clean selection of candidate events particle ID on 4He [scintillators] Pb-glass selection of energetic photons good missing mass (π0) reconstruction scattering angle [WC1] TOF in channel [ΔE2–ΔE1] (include channel energy loss, compensate for PMT time drift) NOTE: cross section normalized to d+p elastic forbidden by: isospin conservation π0 is T = 1 charge symmetry π0 is odd under CS major physics background: d+d→4He+γ+γ

10. Particle Identification (using scintillator signals) ΔE2 E Windows select 4He events but rate is 103 too high due to d-induced reactions on residual gas and beam pipe ΔE1 ΔE2 Select 2-photon events with left and right Pb-glass events inside window Final cut leaves no background, only π0 and γ+γ final cut all events that pass particle identification

11. Results at two energies near threshold σTOT = upper 2 MeV of γ+γ continuum: σ = 6.9 ± 0.9 pb and 9.5 ± 1.4 pb (about twice prediction) π0 peak γ+γ continuum 12.7 ± 2.2 pb (scaled for channel acceptance) results consistent with S-wave peak positions correct to 60 keV σTOT/η 100 systematic errors about 7% (excluding normalization) 15.1 ± 3.1 pb average 50 η = pπ/mπ 0 0.1 0 0.2

12. Cross section normalized to d+p elastic scattering To calibrate online monitor, use HD gas and observe d+p elastic scattering at 25°(d) – 44°(p). [see K. Ermisch, PRC 71, 064004] 108 MeV 120 MeV 135 MeV interpolate to 116 MeV online monitor is d+d elastic at 90°cm

13. but the KVI measurements disagree with Japanese data [Sekiguchi, PRL 95, 162301] Energy dependence We need the cross section here. 116 MeV Ermisch data new RCNP data compare Other measurements on graph: 93.6 MeV: Chamberlain/Stern, PR 94, 666 (’54) 146 MeV: Postma/Wilson, PR 121, 1229 (’61) 155 MeV: Kuroda et al., NP 88, 33 (’66) 198 MeV: Adelberger/Brown, PR 5, 2139 (’72) KVI data More work is needed! Japanese data

14. Charge Symmetry Breaking contributions from Effective Field Theory Nucleons and pions are components of model. van Kolck, Niskanen, and Miller, PL B 493 (2000) 65 Scale parameters determined from experiment. Leading order contributions: Down-up quark mass difference Electro- magnetic nucleon-only contribution nucleon-pion scattering (free pion-nucleon system limited to π+ or π– with protons; large EM corrections limit view of CSB) Estimate size: Cottingham sum rule: π0 In π0 production, consider: x so: CSB here

15. Theory status (work in progress): Pion rescattering large for n+p→d+π0, but vanishes (except recoil and ISI) for d+d→4He+π0. Other terms matter! Re-introduce meson- exchange and meson mixing to stand in for missing EFT terms. Add π0–η mixing. n+p→d+π0 d+d→4He+π van Kolck prediction: re-evaluation: [contributions to amplitude] pb % [S, P-wave interference] 0.5 50 data data 0 0 ρ0–ω mixing n-p mass difference π0–η mixing EFT EM π0–η mixing (Niskanen) EFT quark mass difference π0–η mixing downscaled wavefunction isospin mixing EFT pion rescattering

16. Further comments: Pion production experiments and EFT theory happened together. After several attempts, experiments have succeeded. New approaches achieved required sensitivity. Interpretation is still in progress: EFT has focused us on quark origins of CSB, including meson mixing. Even at threshold, pion production required high momentum transfer. This does not fit easily into EFT expansion scheme. Next order EFT large, make quantitative by bringing meson exchange back. Input still not well controlled (strength of eta-nucleon coupling). Theory sensitive to all ingredients: wavefunctions (high p), isospin mixing, ISI. There may be more: excite deuteron to T=1 state at beginning. Four-body calculations just beginning. Experiment in 2002 on d+d elastic (cross section and analyzing power) at IUCF. Further extension to reaction channels at KVI.

17. New experimental efforts at COSY (with WASA 4πdetector): (further work on d+d→4He+π0) Get P-wave from higher energies (new number for EFT). But new CS allowed channels (p+t+π0, n+3He+π0) open. Examine region of a0–f0 to explore mixing.

18. Teams working on the “new” charge symmetry: Many thanks to… Theory A. Gårdestig C.J. Horowitz A. Nogga A.C. Fonseca C. Hanhart G.A. Miller J.A. Niskanen U. van Kolck TRIUMF group A.K. Opper E. Korkmaz D.A. Hutcheon R. Abegg C.A. Davis R.W. Finlay P.W. Green L.G. Greeniaus D.V. Jordan J.A. Niskanen G.V. O’Reilly T.A. Percelli S.D. Reitzner P.L. Walden S. Yen IUCF group E.J. Stephenson A.D. Bacher C.E. Allgower A. Gårdestig C.M. Lavelle G.A. Miller H. Nann J. Olmsted P.V. Pancella M.A. Pickar J. Rapaport T. Rinckel A. Smith H.M. Spinka U. van Kolck elastic analysis A. Micherdzinska